Laminated sheets for printed circuit boards are known which have a prescribed number of stacked prepregs, each obtained by impregnating a fiber base material with an electrical insulating resin composition, heated and pressed into an integral form. Also, metal-clad laminates are used when forming printed circuits by a subtractive process. Such metal-clad laminates are manufactured by stacking metal foil such as copper foil on the prepreg surface (one or both sides), and heating and pressing the stack. Thermosetting resins such as phenol resins, epoxy resins, polyimide resins and bismaleimide-triazine resins are widely used as electrical insulating resins, and thermoplastic resins such as fluorine resins and polyphenylene ether resins are also sometimes used.
However, the advancing development of data terminal devices such as personal computers and cellular phones has led to reduced sizes and higher densities of the printed circuit boards mounted therein. The forms in which electronic components are mounted in printed circuit boards range from pin insertion types to surface mounting types, and are gradually shifting toward area arrays such as BGA (ball grid arrays) that employ plastic boards. For a substrate on which a bare chip such as BGA is directly mounted, connection between the chip and substrate is usually accomplished by wire bonding which employs thermosonic bonding. Bare chip-mounted substrates are thus exposed to high temperatures of 150° C. and above, and the electrical insulating resins must therefore have a certain degree of heat resistance.
With trends toward lead-free solder from the viewpoint of environmental concerns, the higher solder melting temperatures are resulting in demand for greater heat resistance for substrates, while demands for halogen-free materials are also increasing, such that it is becoming difficult to use bromine-based flame retardants.
Repairability, which allows removal of mounted chips, is also often in demand, and this requires approximately the same amount of heat as for mounting of the chips, as the chip must be remounted later on the substrate and subjected to further heat treatment. Conventional insulating resin systems have also sometimes caused peeling between the resins and fiber base materials. Consequently, “repairable” substrates must exhibit heat resistance at high temperatures.
There have been proposed prepregs comprising a fiber base material impregnated with a resin composition with a thermosetting resin such as an epoxy resin, and a polyamideimide as essential components, in order to obtain excellent heat resistance and improve the fine-pitch wiring formability (see Patent document 1, for example). However, resin compositions comprising a polyamideimide and an epoxy resin, while exhibiting high bonding strength for various adherends and having excellent heat resistance, also have strong cohesion between molecules and therefore exhibit high melt viscosity during hot molding and thus inferior moldability. Methods have therefore been proposed in which moldability is obtained by low molecularization of the polyamideimide molecular weight to a degree that permits molding, or by combination of an epoxy resin that exhibits low melt viscosity (see Patent documents 2 and 3, for example).